I think all the elements are equally lethal in combat, and I think they do a pretty good job portraying that.

Lighting = Razor Sharp Water Attack = Boulder = Air Blast Of Doom.

However, since the only airbending we've really seen is by Aang and like all airbenders he is a pacifist, airbending doesn't really appear as lethal as say fire-bending which is mostly portrayed in negative, violent connotations through ATLA and also a bit in LOK. The same is pretty true for earth bending. I really think it depends on the skill of the bender, if an extra technique is involved (lightening, metal, blood) or if the bender is trying to kill their opponent.

In terms of survival, I'd say waterbending since survival is all about adapting and that's pretty much the description of waterbending.